I'm not sure how you looked up bottom bracket without coming across Sheldon's the or Park Tool sites, both of which have very full explanations with pictures of bottom bracket specifications, tools and adjust/repair procedures. There's no need to repeat everything here, so just Google (replace/overhaul/adjust/fix) bottom bracket and hit the YouTube option for videos.

As for size, you don't even know if you need to replace anything until you disassemble, at which point you have the parts in your hand to measure or take to a shop.